// 148ms
public ListNode ReverseListWithStack(ListNode head)
{
ListNode newHead = head, ptr = head;
Stack<ListNode> s = new Stack<ListNode>();
while (ptr != null)
{
s.Push(ptr);
ptr = ptr.next;
}
int cnt = s.Count;
if (cnt > 0)
{
newHead = s.Pop();
ptr = newHead;
for (int i = 1; i < cnt; i++)
{
ptr.next = s.Pop();
ptr = ptr.next;
}
ptr.next = null;
}
return newHead;
}
public ListNode ReverseListRecursion(ListNode head)
{
if (head == null) return null;
else
{
Reverse(head);
return newHead;
}
}
public static void print(ListNode head)
{
ListNode ptr = head;
while (ptr != null)
{
Console.Write(ptr.val);
ptr = ptr.next;
}
}
static void Main(string[] args)
{
ListNode head = new ListNode(0);
ListNode n1 = new ListNode(1);
ListNode n2 = new ListNode(2);
head.next = n1;
n1.next = n2;
Program p = new Program();
ListNode newHead = p.ReverseListRecursion(head);
Program.print(newHead);
Console.ReadKey();
}
private ListNode Reverse(ListNode node)
{
// ptr point to the last element after reverse
ListNode ptr;
//base case
if (node.next == null)
{
newHead = node;
}
else
{
ptr = Reverse(node.next);
ptr.next = node;
node.next = null;
}
return node;
}